She called the separation policy “cruel” and “immoral” in a guest column for The Washington Post on Sunday.

Laura Bush condemned the new crackdown (Carl Court/PA)

Current First Lady Melania Trump, who has tended to stay out of contentious policy debates, has waded into the issue.

Her spokeswoman said Mrs Trump believes “we need to be a country that follows all laws”, but also one “that governs with heart”.

“Mrs Trump hates to see children separated from their families and hopes both sides of the aisle can finally come together to achieve successful immigration reform,” spokeswoman Stephanie Grisham said.

Another former first lady, former US secretary of State Hillary Clinton called the “zero tolerance” policy “a moral and humanitarian crisis”.

Speaking at an awards lunch for the Women’s Forum of New York, Mrs Clinton said what was happening to families at the US-Mexico border is “horrific”.

“Every human being with a sense of compassion and decency should be outraged,” Mrs Clinton said.